The invention is concerned with an apparatus for the cutaneous determination of the partial pressure of gases in blood with a heated polarographic blood gas electrode and an arrangement for the determination of the blood gas availability at the contact surface. The term "cutaneous" used herein denotes a bloodless measurement recording by a an electrochemical sensor placed on the skin.
It is well known that the pO.sub.2 -value measured with heated cutaneous oxygen electrodes does not always correlate with the arterial pO.sub.2 -value, since other parameters such as, for example, the microcirculation, diffusion resistance of the skin and the circulatory condition can also influence the cutaneous pO.sub.2 -value. Thus, for example, some medicaments which have vasodilating or vasoconstricting activity, used in intensive care cause fluctuations in the cutaneous pO.sub.2 -value while the central-arterial pO.sub.2 -value remains constant. Likewise, it is known that fluctuations in the blood pressure (especially in the case of patients with a variable circulatory condition) can influence the cutaneous pO.sub.2 -value. For these reasons a satisfactory interpretation of the cutaneous pO.sub.2 -value is not always possible. This restriction has contributed materially to the fact that the method for cutaneous pO.sub.2 -measurement in the supervision of adult intensive care patients has hitherto not succeeded.
Experiments have already been carried out to couple a perfusion measurement with the cutaneous pO.sub.2 -measurement in order to enable an improved interpretation of the cutaneous pO.sub.2 -value. This has hitherto exclusively been carried out using methods which rely on the determination of the heat transport at the measurement position. The method described by Lubbers and co-workers, U.S. Pat. No. 3,918,434, relies on the measurement of the heat energy which is required to heat the oxygen electrode to a constant temperature. With increased local perfusion a greater heat energy is consumed. One disadvantage of this method is, however, that only a small heat energy (according to recently published estimates 25%) is removed by the blood flow. The rest of the heat energy is used to heat non-perfused tissues as well as the surroundings of the electrode. For this reason the method according to Lubbers et al. is relatively unsensitive and, moreover, requires a high expenditure for the heat isolation of the electrode against the surroundings. It is, however, accepted that in principle the validity of the cutaneous pO.sub.2 -observation is increased by a simultaneous measurement of the perfusion.
Apart from the method described by Lubbers by determining the heat energy for the maintenance of a constant temperature at the measurement position there are still other methods which rely on the measurement of the heat transport. Another method consists, for example, in applying heat at one position and measuring the temperature at a second position situated at a determined distance therefrom. The temperature difference produced is likewise a measurement of the perfusion.
The previous discussions of the state of the art relate almost exclusively to the measurement of the pO.sub.2 in blood. This is attributed to the fact that until now only the pO.sub.2 -measurement has found acceptance in clinical practice. It is, however, known to the person skilled in the art that the partial pressure of other blood gases, especially the pCO.sub.2, can be measured in principally the same manner. In so doing, such measurements can be carried out either separately or combined with one another. With regard to this, the present invention is not limited to the measurement of a particular blood gas concentration; it can be used not only in the measurement of pO.sub.2 but also of other gases in blood which are to be determined electro-chemically, especially the pCO.sub.2.